Usually, one of the first questions I get asked when I get hire for a location portrait is “What to Wear” I just added a basic overview to my website to help with this question. There is not a perfect answer because it really involves a combination of factors. Location, hair color, purpose and a variety of other factors. Most important is probably location. Color or patterns in clothing can make or break a photo. A general rule is patterns are very difficult in rural locations unless the background is very neutral. Patterns and bold color work best in urban or neutral location.
The best scenario is to consult with the photographer. They can give you suggestions on what to wear. A variety of clothing is best.
Click on any of the above links to go directly to my page with examples of how clothing can affect a portrait.